1. Field of the Invention
The present invention relates to an air-fuel ratio control device for controlling the air-fuel ratio of an engine.
2. Description of Related Art
The use of a catalytic converter to reduce toxic emissions expelled in the exhaust of a motor vehicle engine is well known. Conventionally, a catalytic converter is located in an exhaust passage of the vehicle, the exhaust passage conveying exhaust downstream away from the engine. If the catalytic converter is disposed in the exhaust passage at a distance away from the engine, it takes a significant time for the catalyst within the catalytic converter to be activated sufficiently by the heat of the exhaust to effectively purify the exhaust. However, if the catalytic converter is disposed in the exhaust passage near the engine, the durability of the catalyst is degraded by too much heat.
Previous systems have attempted to solve these problems by using a main catalytic converter disposed in a main exhaust passage, combined with a bypass catalytic converter disposed in a bypass passage in parallel with a portion of the main passage. The bypass passage branches away from and rejoins the main passage upstream of the main catalytic converter. A switching valve disposed in the main passage upstream of the main catalytic converter directs exhaust through the main or bypass passages. Disposed in the main passage downstream of the switching valve is a main air-fuel ratio sensor having a heated sensor element, the main air-fuel ratio sensor providing a signal based upon which the air-fuel ratio of the engine can be controlled. Until the main catalytic converter is activated, the switching valve closes the main passage so that exhaust flows in the bypass passage. This makes it possible for exhaust flowing in the bypass passage to be purified by the bypass catalytic converter, which is activated earlier than the main catalytic converter, thus improving the efficiency and preventing the degradation of purification of the exhaust.
While the switching valve is closed, blocking exhaust flow in the main passage, some exhaust from the engine may linger in a deadheaded portion the main passage upstream of the switching valve. The lingering gas cools so that the temperature of the lingering gas becomes lower than that of exhaust immediately after expulsion from the engine. As the lingering gas is cooled, moisture in the lingering gas condenses. If this moisture flows downstream during the opening of the switching valve and contacts the main air-fuel ratio sensor disposed downstream in the main passage, the air-fuel ratio sensor is suddenly cooled and the heated sensor element may crack or be otherwise damaged. Therefore, until the moisture in lingering gas has passed the main air-fuel ratio sensor disposed in the main passage, the air-fuel ratio is controlled based on a value detected by a bypass air-fuel sensor disposed in the bypass passage. After the moisture has passed beyond the main air-fuel ratio sensor, the main air-fuel ratio sensor element is heated to an activating temperature, and the air-fuel ratio is controlled based on a value detected by the main air-fuel ratio sensor. Thus, cracking of the sensor element of the main air-fuel ratio sensor is prevented.
However, when the switching valve opens, most of the exhaust expelled from the engine flows through the main passage, the main passage having a larger cross-sectional area than the bypass passage. As a result, the exhaust flow rate in the bypass passage is decreased, so that the quantity of exhaust flowing to the bypass air-fuel ratio sensor becomes insufficient and hence the responsiveness of the bypass air-fuel ratio sensor disposed in the bypass passage decreases. In particular, at low flow rates, the bypass air-fuel ratio sensor lags in detecting changes in the actual air-fuel ratio of the engine. Accordingly, while awaiting passage of the moisture beyond the main air-fuel ratio sensor after the switching valve opens, controlling the air-fuel ratio of the engine based on the value detected by the bypass air-fuel ratio sensor may cause more changes than necessary to the air-fuel ratio. As a result, purification of the exhaust may be poor during this time period.
In sum, when the bypass air-fuel ratio sensor is used while the switching valve is open, most of the exhaust flows through the main passage, resulting in a decreased quantity of exhaust flow to the bypass air-fuel ratio sensor, and hence a lag in the responsiveness of the bypass air-fuel sensor. In this situation, the air-fuel ratio control based on the bypass air-fuel ratio sensor may be poor, leading to deterioration in emission purification.